The invention relates generally to renewable energy sources and more specifically to the use of energy storage in conjunction with renewable energy sources. The invention additionally relates to grid frequency stabilization of grid connected renewable energy sources.
Wind-turbine generators (WTGs) and wind-plants are typically designed to deliver constant active and reactive power to a utility grid with the delivered power being independent of system frequency. This is accomplished by decoupling the rotor inertia and speed from the grid using fast acting power electronics and controls. Due to increases in wind-plant size and penetration, some utilities are now requiring that wind-plant and wind-turbine controls provide enhanced capabilities such as frequency stabilization.
At present wind turbines do not provide frequency regulation services, which are quite essential for maintaining stability (power balance) of the electric power system. Another factor for improving power system stability in wind turbines is inertia response. Generally, inertia controllers use rotor kinetic energy in response to frequency deviations on the grid. For example, if the frequency of the grid goes below the nominal value, the kinetic energy of the rotor is used to export electric power to the grid. This results in the slowing down of the rotor. Similarly, if the frequency of the grid increases above the nominal value, then the kinetic energy of the rotor is increased by importing electric power from the grid. This results in the speeding up of the rotor. Since there is a continuous dithering of frequency on the grid, the inertia controller in response to the frequency tends to cause the rotor speed and/or acceleration to dither. If the rotor mode of oscillation is in the proximity of the grid mode of oscillation, the coupled behavior may excite the resonance. Therefore, the use of rotor inertia for continuous frequency response may not be always possible. Thus, it would be desirable to have a system and method to efficiently address variability of renewable energy to help satisfy the related utility company requirements.